A wide variety of doors are used in industrial settings. Such industrial doors include conventional sectional doors, comprising a series of panels hinged together to form the door. Typically, such a sectional door is movable between doorway-blocking positions and overhead-storing positions. For this purpose, a curved guide track is disposed on either side of the doorway, with one leg (adjacent to the doorway opening) extending vertically along the doorway, and the second leg (projecting back from the doorway opening) disposed above and behind the doorway. A curved track section joins the two legs. Sectional doors may also be vertically stored, that is they may have straight tracks and be movable in a continuous plane between doorway-blocking and doorway-opening positions. Typically, the panels comprising such a sectional door are formed of either metal or wood.
In other types of industrial doors, the door itself may be formed of fabric. One such type of fabric industrial door is a roll-up door, in which the door is a curtain of fabric rolled on a roller tube typically disposed above the doorway opening. To close the door, the curtain is drawn off of the roller, and the roller is reversed to roll the curtain up on the roller for the purpose of opening the door. Another type of fabric industrial door is a so-called "concertina" door. In a concertina door, the door typically also comprises a fabric curtain and a roller is disposed above the doorway opening. Straps are wound onto and off of the roller, and are connected typically to a leading edge of the curtain for the purpose of drawing the curtain up out of the way of the door opening, and allowing the curtain to fall and unfold to cover the door opening. The curtain itself does not wind on the tube, however, and rather is gathered in folds at the top of the door. A still further type of fabric industrial door is a sheet of fabric that is maintained in a flat orientation, and is moved between a doorway-blocking and a doorway-opening position. The doorway opening position may either be above the doorway or overhead, similar to an overhead-style sectional door. Further, while all of these doors have been described as moving vertically, they may also be modified such that their movement between door opening and door closing positions is in a horizontal or other direction, as opposed to a vertical direction. Roll-up doors comprising metal or chain sections are also known.
All of the industrial doors just described typically share a common feature of having a member extending across the doorway opening when the doorway is either in the closed or any of a variety of doorway-blocking positions. Such structure will be referred to herein as an "extension member." In sectional doors, the extension members are the door panels themselves. Fabric doors typically include either a relatively rigid bottom bar extending across the leading edge of the door, and/or other relatively rigid bars extending across the width of the door at locations other than at the leading edge (these are often referred to as "wind bars" as they assist in adding stability to the door and preventing it from billowing under wind load conditions).
A common problem associated with such doors as a result of the presence of these extension members extending across the doorway is unintentional impact. In an industrial or warehouse setting, such impact may occur by virtue of a fork lift or other material handling equipment (or a load carried thereon) being driven into the door and the impact is thus directly or indirectly transmitted to the extension members. In situations where such doors are used in automated factories, conveyors or other devices may cause objects being conveyed to accidentally impact the doors. Given the fact that such impacts are bound to occur, certain types of industrial doors can be designed to withstand such impacts. For example, the panels comprising a sectional door, and the associated track and hardware can be reinforced to withstand such impacts. Of course, this adds significant expense to the door. Further, fabric-type industrial doors typically do not have this option as the door itself is formed of fabric which is more easily damaged than the metal or wood typically comprising sectional doors.
Accordingly, another solution to allowing industrial doors to withstand impact is to allow them to have a controlled breakaway under such an impact. That is, the door is designed for certain components to separate upon an unintentional impact, thus protecting the remainder of the door. One example of such a structure is shown in U.S. Pat. No. 4,676,293, assigned to the assignee of the present invention. In that patent, a sectional door is disclosed that includes a bottom panel having significant flexibility, thus allowing it to absorb impact. However, if the impact is above a predetermined magnitude, the door is designed to allow the roller associated with the bottom-most panel to disengage from the panel, thus allowing the panel to swing free relative to the rest of the door. This action protects the bottom panel from damage.
The various breakaway mechanisms disclosed in the prior art may adequately perform the desired function, but may be limited in use to certain environments or types of doors. In other circumstances, the disclosed breakaway mechanisms may not adequately function. Even when they do function properly, however, reassembly or repair of such doors following breakaway may be a cumbersome or time-consuming process. Spare parts may need to be maintained on hand, and trained technicians may need to be called to reassemble the door following breakaway. Complex breakaway mechanisms may also significantly increase the costs associated with a given door.